More than 60 percent of Twitter’s U.S. users don’t return a month later, based on an analysis unveiled this week by the research firm Nielsen Online.

Twitter allows anyone to write about what they’re doing in messages limited to 140 characters.

Despite the defectors, Twitter is amassing an impressive audience. In March, the San Francisco-based service attracted a U.S. audience of 13.9 million, an increase of more than 25-fold from roughly 500,000 users at the same 2008 juncture, Nielsen said.

“Twitter has enjoyed a nice ride over the last few months, but it will not be able to sustain its meteoric rise without establishing a higher level of user loyalty,” wrote David Martin, Nielsen Online’s vice president of primary research.

Some Web users ditching newspapers

NEW YORK — Sure, plenty of readers are turning more to the Web for newspaper and magazine stories, but are they giving up on print altogether?

In many cases, yes, according to a recent study by the University of Southern California’s Annenberg School for Communication. It found that 22 percent of Internet users have canceled a print subscription because they could get the same product online.

Not that nostalgia for the printed page has died altogether. The survey found that 61 percent of Internet users who read newspapers offline would miss the print edition if it disappeared. That’s up from 56 percent a year earlier.

Newspaper sales slid 7.1 percent in the six months from October to March from the same period a year earlier, according to the Audit Bureau of Circulations.

Doctors advised to add to Wikipedia

NEW YORK — Researchers are suggesting that doctors could be spending more time writing and editing Wikipedia pages on medical topics, despite questions that have been raised about the collaborative online encyclopedia’s credibility.

Medical professionals should recognize that Wikipedia has become a major online source of health information for consumers, researchers wrote in the Journal of the American Medical Informatics Association.

The authors found that, in 71 percent to 85 percent of search words tested at various search engines, Wikipedia came up within the first 10 results.